Dialysis is a well-known method for treatment of kidney insufficiency. In hemodialysis, the blood of a patient suffering from impaired kidney function is conducted from a patient blood vessel to a dialysis machine and is returned to the patient after the treatment. The blood is conducted along one side of a permeable membrane in a dialyzer or filter connected to the dialysis machine, at the same time as dialysis fluid or dialysate may be conducted along the opposite side of the same membrane. Waste substances or poisons that are to be removed from the blood pass potentially with the help of diffusion from the blood to the dialysis fluid through the membrane. Excess water is also removed from the blood. A hemodialysis treatment typically lasts 3-5 hours. Preferably, a treatment may be performed while the patient is sleeping during night and may in that case last for about 8 hours.
On-line preparation of dialysis fluid is known. For example, the on-line preparation of a saturated bicarbonate solution from powdered bicarbonate contained in a cartridge is disclosed in DE 198 01 107 A1.
For the preparation of a dialysis fluid, saturated sodium bicarbonate solution may be mixed with a solution comprising appropriate electrolytes, such as K+, Ca2+ and Mg2+.
In EP 0 278 100 B1 a powder cartridge is shown. The cartridge comprises a closed vessel provided with penetrable membranes at its upper inlet end and its lower outlet end, respectively. Within the vessel, there is provided a supply of powder concentrate of sufficient quantity so as to be suitable for a dialysis treatment session. For instance, in connection with the preparation of dialysis fluid or solution, the concentrate may consist of powdered sodium chloride or sodium bicarbonate.
In use, a powder containing prior-art cartridge is first primed with fluid, such as water, either from the top or from the bottom. Enough fluid is introduced into the cartridge during priming so that the fluid level is above the powder level. The powder is dissolved in the fluid and a saturated solution can leave the cartridge through the outlet in the bottom of the cartridge. As the solution leaves the cartridge a corresponding amount of new fluid is introduced into the cartridge. To establish that the degree of saturation of the solution leaving the cartridge is satisfactory, the conductivity of the solution may be measured. An unsatisfactory conductivity (saturation) will trigger an alarm in the dialysis machine.
The above-mentioned way of on-line preparation of solutions for medical use by means of a powder cartridge has many advantages. There are, however, some disadvantages with prior-art cartridges. After a few hours of use, the mixture of the fluid and the powder in the cartridge may become inhomogeneous. In some instances, clods may be formed. The clods may prevent gas bubbles from rising to the liquid surface. When a bubble is “released”, a channel in the powder bed may be created, sometimes all the way down to the lower outlet of the cartridge. This may allow unsaturated solution to leave the cartridge causing a conductivity alarm. The alarm must be taken care of either by the patient or by a nurse or medical attendant. If these problems occur, they may be remedied by knocking on the cartridge wall or by shaking the cartridge. However, often the cartridge has to be discarded, while still containing a considerable amount of powder, and replaced by a new cartridge.
The formation of channels in the powder bed might have other causes. For example, the fluid may be introduced gradually through the top inlet and fall in drops down to and impact the liquid surface. This may give rise to pressure waves in the liquid bed which in turn create fluid currents perpendicular to the powder bed surface. These currents may work their way down in the powder bed, creating channels in the bed reaching the lower outlet, making it possible for unsaturated solution to leave the cartridge.